1. Field of the Invention
This invention relates to an optical modulator and an optical waveguide device suitable for use with an optical communication system.
2. Description of the Related Art
FIG. 16 is a schematic view showing a conventional optical multiplexer/demultiplexer having two inputs and two outputs. The optical multiplexer/demultiplexer 700 shown in FIG. 16 includes two input waveguides 702 and 703, a waveguide coupling element 706 and two output waveguides 704 and 705 formed on a substrate 701. The optical multiplexer/demultiplexer 700 can multiplex or demultiplex, when input light propagating in the input waveguides 702 and/or 703 is to be outputted to the output waveguides 704 or/and 705 through the waveguide coupling element 706, the input light.
In the optical multiplexer/demultiplexer 700, for example, input light P0 inputted to the input waveguide 702 is demultiplexed into and outputted as output lights P1 and P2 from the output waveguides 704 and 705 through the waveguide coupling element 706.
Here, in FIG. 16, the input and output waveguides 702 to 705 have an equal width w0, and they are connected with a width d left therebetween to the waveguide coupling element 706. Further, the two input waveguides 702 and 703 and the two output waveguides 704 and 705 are formed individually with an angle θ defined therebetween such that the distance therebetween decreases toward the waveguide coupling element 706.
The connection angle θ of the input and output waveguides 702 to 705 to the interference waveguide 706 is sufficiently small. Therefore, since the widths A and B of the connection points between the input and output waveguides 702 to 705 and the waveguide coupling element 706 can be approximated as an equal value to the input and output waveguide width w0, the width ww of the waveguide coupling element 706 can be represented as 2w0+d. It is to be noted that, in FIG. 16, reference character Lc denotes the length of the waveguide coupling element 706 in its longitudinal direction (propagation direction of light).
For example, if the length Lc is adjusted to form the waveguide coupling element 706 as shown in FIG. 17, then the characteristic as the optical multiplexer/demultiplexer 700, that is, the multiplexing/demultiplexing characteristic for input light, can be varied.
In particular, as shown in FIG. 17, where the length Lc is equal to L1, the ratio [P1/{P1+P2}] of the output light P1 to the overall output light {P1+P2} is equal to 1. Therefore, the output light from the optical multiplexer/demultiplexer 700 is outputted as the output light P1 only from the output waveguide 704.
On the other hand, where the length Lc is equal to L2, since the ratio P1/{P1+P2} is equal to 0.5, the branching ratio of the input light P0 at the output waveguides 704 and 705 is 1:1, and the output light from the optical multiplexer/demultiplexer 700 is outputted as the output lights P1 and P2 from the output waveguides 704 and 705, respectively. In other words, the optical multiplexer/demultiplexer 700 can be used as a 3 dB coupler.
Further, where the length Lc is equal to L3, since the ratio P1/{P1+P2} is equal to 0, the output light from the optical multiplexer/demultiplexer 700 is outputted as the output light P2 only from the output waveguide 705.
The optical multiplexer/demultiplexer 700 having such a configuration as described above can be used as 3 dB couplers 700A and 700B, for example, in such a Mach-Zehnder type modulator 710 as shown in FIG. 18. Here, the Mach-Zehnder type optical modulator 710 shown in FIG. 18 includes a Mach-Zehnder type optical waveguide 714 formed from 3 dB couplers 700A and 700B and two linear waveguides 712 and 713 on a substrate 711, and further includes a signal electrode 715 and a ground electrode 716 formed on the substrate 711.
In the Mach-Zehnder type optical modulator 710 having the configuration described above, input light inputted through an input waveguide 702A from between two input waveguides 702A and 703A which form the 3 dB coupler 700A is branched at a ratio of 1:1, and the branched lights propagate in the linear optical waveguides 712 and 713. It is to be noted that, by a variation of an electric field by a signal voltage applied to the signal electrode 715, light modulated in accordance with the signal voltage just mentioned can be outputted from the output waveguides 704B and 705B.
Further, the lights propagating in the linear waveguides 712 and 713 are inputted to the input waveguides 702B and 703B which form the 3 dB coupler 700B, respectively, and then multiplexed by the waveguide coupling element 706B, whereafter the multiplexed light is outputted from both of the output waveguides 704B and 705B. Particularly, the output light from the output waveguide 704B is used as monitor light, and the output light from the output waveguide 705B is used as signal light.
However, in such a conventional optical waveguide device as described above, a great difference exists between a propagation mode in the input and output waveguides and a propagation mode in the waveguide coupling element, and therefore, there is a subject that an radiation mode appear.
Further, also where an optical waveguide device from which such radiation mode light as just described is generated is applied to a device such as an optical modulator, a wavelength filter or the like, there is the possibility that, by generation of the radiation mode described above, the performance improvement of the device itself may be hindered.
Particularly, where the optical waveguide device 700 having such a configuration as described above is applied as the 3 dB couplers 700A and 700B to such a Mach-Zehnder type optical modulator 700 as described above with reference to FIG. 18, radiation mode light is generated in the 3 dB coupler 700B described above. Therefore, a deviation δ appears between a voltage value V1 when the signal light (refer to a full line waveform in FIG. 19) is in an on state and a voltage value V2 at an off point of the monitor light (refer to a broken line waveform in FIG. 19) as shown in FIG. 19 although they are desirably to be coincident with each other. The ratio [(V2−V1)/Vπ] of the deviation to the half-wavelength voltage Vπ is approximately 10%, and this makes a hindrance to use of the output light of the conventional optical waveguide device 700 as monitor light for adjustment of the operating point.